With the terms “granules” or “granular”, it is intended to indicate in the present description and in the claims the small scales, sheets or plates produced by the grinding-crushing of slab, sheet, film and the like plastic material. In the plants for working and/or transforming plastic materials reduced into granules, granular material is transported from a storage container to one or more machines designed to use such a material and usually comprising injection or thermoforming presses, by means of a pneumatic conveyance or transport system, preferably operating under reduced pressure. The transport system must ensure a minimum flow rate of granular material, thereby ensuring a continuous feed of granular material to the transformation machine or machines.
In reduced-pressure transportation systems of granular material proposed up to now, a vacuum source is provided, e. g. a vacuum pump, arranged to suck air from a container of granular plastic material. The granular material is thus driven by the suctioned air along a suction tubing which leads above, and discharges the granular material into, a collection tank, whereas the transport air is suctioned to convey towards the vacuum source. Between the collection tank of the granular material and the vacuum source, a filter is provided to filter the air, which has just separated from the bulk of the granular material, before it reaches the vacuum source. An electronic control unit controls the entire cycle. As a matter of fact, it is the atmospheric pressure that pushes the granular material along the tubing towards the vacuum source.
For a correct conveyance of the granular plastic material within the ducts or tubing, the air flow created by the vacuum source must flow within a desired speed range, both to prevent the material from being conveyed at overly high speeds deemed “dangerous”, and to prevent the stagnation of the granular material if the conveyance speed is not sufficiently high.
One of the most difficult problems to solve in the reduced pressure transport of granular material within conveyance ducts is that of being suitable for maintaining its transfer speed constant, even with the change of light or section of the ducts and/or configuration (curved, rectilinear) of the tubes along which the conveyance is carried out.
In the conventional plants, and in particular along the transport tubing, the speed of a granular material is usually not maintained constant over time. In the various conveyance steps carried out in a conventional reduced-pressure transport plant, the conveyed plastic material granules usually reach very high speeds, even double the optimum speed. When high speeds are reached, plastic material granules scrape against the walls, especially at the curved tubing sections, and due to the combined effect both of the centrifugal force and the electrostatic charges and to the friction they tend to adhere to the walls and to form thin film encrustations or deposits on the walls themselves. Such deposits, after a certain lapse of plant functioning time, are detached from the tubing walls, giving rise to multilayer crusts or scales of materials that are even different from each other, considering that they are usually fed in different cycles through one same tubing. The multilayer crusts or scales that are detached from the walls constitute a source of pollution/contamination for the granular materials that are conveyed along the tubing after their detachment from the inner wall of the tubing itself. This phenomenon is called “angel hair” formation in jargon.